Increasingly high requirements are being set for magnetic recording media with regard to recording, playback and aging resistance. The binder is becoming increasingly important for meeting these requirements.
For example, to improve the magnetic properties, in particular the residual induction, a high packing density of the magnetic material in the layer is desirable, which results in a reduction in the binder content of the layer. Attempts have also been made to achieve an improved signal/noise ratio and a very narrow field strength distribution by using increasingly finely divided magnetic materials having a pronounced and extremely uniform acicular shape. These materials are also very often surface-modified to reduce aging phenomena. Because of such measures, both the division of the pigments in the dispersing process and the achievement of good dispersion stability are made considerably more difficult. Moreover, the magnetic layers must be very flexible and have high resilience and a high tensile strength. In addition, to avoid drops in output level, a reduction in the coefficients of friction and an increase in the abrasion resistance and resistance to wear are increasingly required. Furthermore, this mechanical stability of the magnetic layer must be ensured even at high temperatures and high atmospheric humidity.
It is known that magnetic layers which are subjected to severe mechanical stresses contain polyurethane elastomers which prove to be advantageous as binders. Polyesterurethanes as described in DE-B 1 106 959, DE-B 2 753 694, EP-A 0 069 955 or U.S. Pat. No. 2,899,411 have proven particularly useful.
However, these binders can no longer adequately fulfil all of the requirements and measures described above. In many cases, particularly with the very finely divided materials, pigment wetting and pigment dispersing is adversely affected so that any sintered material produced in the milling process is insufficiently divided up, while agglomeration of the pigment particles is not sufficiently prevented, leading to poor magnetic properties and hence to poor tape properties in respect of electroacoustics and video data. For this reason, low molecular weight dispersants are added in relatively small amounts to facilitate the dispersing process. However, these dispersants have disadvantages. For example, low molecular weight dispersants may readily be exuded under disadvantageous climatic conditions, such as high temperature and/or high atmospheric humidity. This gives rise to deposits on all parts in contact with the tape, especially on the head, in recording or playback apparatuses, causing drops in output levels. Furthermore, the friction (adhesion) increases sharply, with the result that the tape may come to a stop, ie. block. When dispersing resins are used, on the other hand, there may be compatibility problems in the dispersion. Since these dispersants contain polar groups, the hydrophilicity of the layer increases greatly, and hence, especially in a humid climate, also effects such as swelling of the layer, exudation of the dispersants and lubricants and changes in the mechanical properties due to changes in the plasticizer effects.
To improve the dispersing properties of the polyurethane binders, it was proposed at an early stage to incorporate polar groups in the binder. These polar groups can in principle be introduced via any component which is used in the preparation of the polyurethane. Polyesters having polar groups are most frequently used (inter alia DE-A 28 33 845). Incorporation of diols which additionally carry polar groups is described in, for example, JP-A 57 092 421, German Laid-Open Application DOS 3,814,536 or EP-A 193 084. Subsequent incorporation of the polar groups by an S.sub.N reaction at the terminal OH group of the polyurethanes is disclosed in JP-A 57 092 422. Although these dispersing resins disperse the pigments very well on the one hand, they lead on the other hand, especially with increasing fineness of the magnetic pigments used, to dispersions having extremely disadvantageous rheological properties, such as a high flow limit and high viscosity, so that further processing of these dispersions to magnetic layers is very difficult. Furthermore, the content of ionic dispersant groups in the dispersing resin is limited to a maximum concentration since otherwise adverse effects, such as flocculation with bridging, occur. An increase in the pigment volume concentration on the one hand and a reduction in the particle size on the other hand are no longer compatible with the dispersing resin content then required.
A substantial improvement in the dispersing behavior could be achieved using low molecular weight, OH-containing polyurethanes, as described in European Patent 0,099,533. However, these measures too are not sufficient for dispersing the increasingly finely divided pigments and for meeting the higher and higher requirements which the magnetic recording media have to meet with respect to mechanical and magnetic properties.